1. Field of the Invention
The present invention relates to an electrical connection structure between a piezoelectric element and a wiring member, as well as to a head suspension employing the electrical connection structure.
2. Description of Related Art
Small-sized precision information devices are rapidly advancing, and for use with such devices, needs for micro-actuators capable of conducting positioning control for very small distances are increasing. Such micro-actuators are highly needed by, for example, optical systems for correcting focuses and inclination angles, ink jet printers for controlling ink heads, and magnetic disk drives for controlling magnetic heads.
The magnetic disk drives increase storage capacities by increasing the number of tracks per inch (TPI), i.e., by narrowing the width of each track on a magnetic disk.
The magnetic disk drive, therefore, needs an actuator capable of precisely positioning the magnetic head within a minute range across tracks.
To meet the need, a head suspension with a dual actuator system is proposed in Japanese Unexamined Patent Application Publication No. 2002-50140. The dual actuator system employs a piezoelectric element in addition to a usual voice coil motor that drives a carriage to which the head suspension is attached. The piezoelectric element is made of piezoelectric ceramics and is arranged between a base plate and a load beam of the head suspension. According to this related art, the voice coil motor turns the head suspension through the carriage, and in addition, the piezoelectric element deforms in proportion to a voltage applied thereto, to minutely move a magnetic head at a front end of the load beam in a sway direction (a widthwise direction of the load beam) relative to the base plate.
With the dual actuator system involving the voice coil motor and piezoelectric element, the related art is capable of precisely positioning the magnetic head to a target position on a magnetic disk.
The head suspension employing the dual actuator system connects an electrode surface of the piezoelectric element and a terminal surface of a wiring member to each other with a conductive adhesive.
To improve the electrical characteristics of connection between the piezoelectric element and the wiring member, the electrode surface of the piezoelectric element is made by plating gold and the terminal surface of the wiring member is covered with a gold plate layer. The gold plate layer has a thickness in the range of about 0.1 to 5.0 μm, for example, a thickness of about 0.5 μm. The gold-plated electrode surface and the gold plate layer on the terminal surface are bonded together with a conductive adhesive.
This technique seems to achieve same bonding conditions on the electrode surface of the piezoelectric element and the terminal surface of the wiring member.
However, a peeling test to peel off the electrode surface of the piezoelectric element and the terminal surface of the wiring member from each other shows that the conductive adhesive is entirely peeled off from the terminal surface of the wiring member and is left on the electrode surface of the piezoelectric element, instead of being broken and separated evenly to the terminal surface and electrode surface.
This means that the original bonding strength of the conductive adhesive is not fully used to bond the terminal surface and electrode surface together, and therefore, the reliability of electrical connection between them is dubious.
FIG. 26 is a photograph showing the terminal surface of the wiring member after the peeling test and FIG. 27 is a photograph showing the electrode surface of the piezoelectric element after the peeling test.
After the peeling test, the terminal surface of the terminal 103 of the wiring member (flexure) 101 clearly exposes the gold plate layer 105 as seen in FIG. 26 and the conductive adhesive 109 remains only on the electrode surface 107a of the piezoelectric element 107 as seen in FIG. 27.
The inventors of the present invention have studied a cause of the conductive adhesive being entirely left on the electrode surface 107a of the piezoelectric element 107, although the terminal surface of the terminal 103 and the electrode surface 107a are similarly plated with gold.
The study has found that the surface texture of the piezoelectric element 107 made of the piezoelectric ceramics is the cause. Namely, the electrode surface 107a of the piezoelectric element 107 is rugged compared with the terminal surface of the terminal 103 of the flexure (wiring member) 101.
Due to this, a bonding strength on the electrode surface 107a of the piezoelectric element 107 plated with gold is stronger than that on the terminal surface of the terminal 103 of the flexure 101 also plated with gold.
To make the surface roughness of the terminal surface of the terminal 103 similar to that of the electrode surface 107a, the terminal surface of the terminal 103 may be processed or scratched with a tool. This, however, deteriorates the electrical characteristics of the terminal 103 or contaminates the terminal 103.